Various proposals have been made for new iron-making processes that substitute existing blast furnace and smelting reduction processes. These proposals relate to the molten metal manufacturing processes for obtaining molten metal, involving pre-reducing metal oxide agglomerates with carbonaceous material in a rotary hearth furnace to form reduced agglomerates and melting the reduced agglomerates in an electric furnace such as an arc furnace or a submerged arc furnace (for example, refer to Patent Literatures 1 to 4).
However, in the existing processes, two steps (a pre-reduction step using a rotary hearth furnace and a melting step using a melting furnace) must be provided. These processes require equipment or facilities for transferring the reduced agglomerates from the rotary hearth furnace to the melting furnace as well as two exhaust gas processing lines, i.e., one for the rotary hearth furnace and one for the melting furnace. Thus, the facility cost increases, the thermal loss increases, and the energy consumption cannot be sufficiently decreased as total system or process.
The inventor of the present invention has performed thorough studies to provide a specific method for manufacturing molten metal in which a rotary hearth furnace is not used and an electric heating furnace only is used to reduce and melt metal oxide agglomerates with carbonaceous material. As a result, the inventor accomplished an invention described below and filed a patent application for the invention (Japanese Patent Application No. 2009-105397; hereafter, the invention of this patent application is referred to as “earlier invention”.)
An apparatus for manufacturing molten metal according to the earlier invention is illustrated in FIGS. 5A and 5B. A stationary non-tilting electric heating furnace, herein, an arc furnace is used that includes raw material charging chutes 4 at both ends 2 of the furnace in the width direction, an electrode 5 in the center position of the furnace in the width direction, and a secondary combustion burner 6 provided in a flat furnace top 1. A carbonaceous material A is charged through the chutes 4 to form a carbonaceous material layer (corresponding to “raw material layer” of the subject invention) 12 having a sloping surface extending downward toward the lower portion of the electrode 5. Metal oxide agglomerates with carbonaceous material B are subsequently charged to form an agglomerate layer (corresponding to “metal agglomerate raw material layer” of the subject invention) 13 on the sloping surface of the carbonaceous material layer 12. Arc heating is then conducted with the electrode 5 to sequentially melt the lower end portion of the agglomerate layer 13 to form a molten metal layer 14 and a molten slag layer 15. At the same time, while the agglomerate layer 13 is allowed to descend along the sloping surface of the carbonaceous material layer 12, the agglomerate layer 13 is heated with radiant heat from secondary combustion by blowing oxygen-containing gas C through the secondary combustion burner 6 to burn CO-containing gas generated from the agglomerate layer 13.
According to the earlier invention, while an agglomerate layer is allowed to move along the sloping surface of a raw material layer formed in a furnace toward an electrode, the agglomerate layer is pre-reduced by heating with radiant heat from secondary combustion by blowing oxygen-containing gas through a secondary combustion burner to burn CO-containing gas generated from the agglomerate layer; and the pre-reduced agglomerate layer is reduced and melted near the electrode by arc heating to form molten metal. Thus, molten metal is directly obtained from metal oxide agglomerates with carbonaceous material by a single process and hence the facility cost and the energy consumption can be considerably decreased, compared with the existing processes.
However, an apparatus for manufacturing molten metal according to the earlier invention needs to be improved in the mixing of CO-containing gas generated in the furnace and the oxygen-containing gas C blown through the secondary combustion burner 6 provided in the flat furnace top 1. Thus, a further increase in the secondary combustion efficiency and ultimately a further increase in the energy efficiency have been demanded.
When a large amount of oxygen-containing gas C is blown from the flat furnace top 1, the gas is brought into contact with the electrode 5 to seriously wear the electrode 5. Accordingly, a partition wall 9 is provided between the electrode 5 and the secondary combustion burner 6. Although the wear of the electrode 5 is suppressed with the partition wall 9, the problem that the partition wall 9 is damaged remains unresolved.
It is difficult to introduce oxygen-containing gas C from an end 2 of the furnace in the width direction because the carbonaceous material layer 12 is present. It is possible to introduce oxygen-containing gas C from an end of the furnace in the longitudinal direction because the gas can be blown into the furnace so as to avoid the carbonaceous material layer 12. However, it is difficult to distribute oxygen-containing gas C over the entirety of the furnace in the longitudinal direction and hence the secondary combustion efficiency becomes poor.
In an apparatus for manufacturing molten metal according to the earlier invention, when agglomerates charged into the furnace have large amounts of powder or agglomerates are sintered or fused together in the furnace, hanging of the agglomerate layer may occur and smooth descent of the agglomerate layer may be inhibited. In this case, agglomerates are not properly reduced or melted by heating and the performance of the apparatus is degraded. When such hanging of the agglomerate layer occurs, it is difficult to provide a mechanical unit that forcedly overcomes the hanging in an apparatus for manufacturing molten metal according to the earlier invention.